1. Field of the Invention
This invention relates to the control of equipment by transmitting digital information over alternating current (AC) power lines and more particularly to apparatus that will receive the digital information in the form of attenuated signals reliably from the AC power lines.
2. Description of the Prior Art
The need for new, more effective control systems for building operations has never been greater. Now more than ever, businesses are turning to advanced central control technology to perform a number of tasks necessary for day-to-day operation. Until recently, however, the complexity and expense of system installation have prevented many building owners and managers from taking advantage of available technology. In many older buildings, it has proven too expensive or even impossible to install these systems because of hard-to-trace wiring or building construction obstacles. Even in newer buildings the cost of running hard control wiring to multiple remote points can be prohibitive. In either case, the costs associated with running wire are the most significant part of the cost of installation, in some buildings as much as 75 percent of the total.
Power line carrier systems effectively eliminate the need for major wiring jobs associated with control system installation. It utilizes the existing in-building AC power service to take the place of hard control wiring. The power line carrier systems can be used in a wide variety of applications, including environmental and energy management control, building and factory automation, remote lighting and process control. There are additional advantages for those who wish to employ control systems in temporary or leased locations. In these cases, the system can be moved easily to a new location or building, eliminating the need to rewire with every move or to leave the control wiring behind.
There are many advantages to using the power line carrier systems instead of hard wiring: installation becomes a much easier task; walls and ceilings are left intact; building occupants and tenants are not inconvenienced by messy installation and repair jobs; and total installation costs are easier to estimate and control.
Power line carrier systems are communication systems geared to pass commands from a master controller, typically a digital computer to energy-using devices. It is desirable to use the existing AC power lines for transmitting these commands between devices by using a radio frequency signal carrier. When large buildings or building complexes are involved, installation of the power line carrier system usually costs much less than installing new wiring between separate energy-using devices and the master controller.
The computer sends digital information to transmitters, typically a Honeywell TC720B transmitter which is described in the Tradeline Catalog No. 70-6910 copyright 1982 and published by the Residential Division, Honeywell Inc., Minneapolis, Minn. 55408.
The transmitter generates frequency shift keyed (FSK) radio signals and applies these signals to standard power lines for transmission to receiver relays, typically Honeywell R720B receiver relays described in the aforementioned Tradeline Catalog.
The transmitter transmits on all these phases of power distribution. The receiver relay receives the FSK radio signals for conversion to binary signals which are applied to a microprocessor. The addressed microprocessor provides output signals to control relays which in turn control the specific devices.
The prior art receiver relays consist of discrete components which are quite costly and have adequate sensitivity of typically 40 millivolts peak-to-peak. However, since the FSK signal being transmitted over the AC power line is attenuated by the varying loads on the power line such as motors, lights, industrial machinery, heating and ventilating equipment, office equipment and computing equipment, it is desirable to have a considerably improved sensitivity.
The power line concept is also applicable to the home environment. U.S. Pat. No. 4,174,517 entitled "Central System for Controlling Remote Devices Over Power Lines" describes such a system. This system, however, does not have the signal attenuation and signal noise problems that the power line carrier system has in its business environment.